During the drilling of a well bore it is often necessary to deflect the drill string, near the drill head, to drill a directional well. On occasions, it is necessary to deflect the drill string to prevent an unwanted influence from urging the progressing bore out of the planned path.
When drilling motors are used near the drill head, or bit, a bent sub can be used to urge the bit to move laterally from the existing well bore. In the past, the bent sub often had to be added to the drill string to influence the well bore direction. Then it had to be removed to progress without the lateral influence. The drill string had to be tripped to change the bent sub.
To eliminate the need for tripping the drill string, many forms of bendable subs have been devised to enable the bending and straightening operation without removing the drill string from the well. Such subs have been beneficial in many cases but often caused problems.
Drilling motors are usually used below the bending apparatus. In very soft formations, jets may be used below the bending apparatus to deflect the progressing well bore.
When used without drilling motors, and in conjunction with selective stabilizer placements, enlargement of some length of well bore has been achieved.